The alternator shaft (in your car or simple aircraft) has a magnet on it which rotates. In so doing it causes a continuously changing magnetic field to pass through the coils surrounding the shaft. This will cause a voltage to appear in these coils which itself varies (alternates). In most practical alternators, this magnetic field is produced by a rotating coil of wire fixed on the shaft (i.e. an electromagnet). The magnetic field it provides must be constant hence the current must pass through this coil in one direction only (D.C.) Direct current is produced by batteries and used by the systems on cars and light aircraft. It is also needed to charge these batteries so the alternating current produced by the varying voltage generated in the coils surrounding this rotating magnet is converted into direct current by passing it through rectifiers. To get the alternator going, dierct current must be provided by the battery, however once it is generating, the D.C. it produces can be used to generate the required magnetic field (i.e. it self-excites.) Anybody who has had to hand swing a light A/C due to a flat battery will know that if there is a little life in the battery, the alternator will start working and the ammeter shows a charging current flowing. If the battery is totally "dead" however the alternator does not work, the ammeter needle stubbornly remains on zero and an immediate shut town is the order of the day so that the battery can be replaced or recharged. If a battery fails in flight however, the electrical system will often continue to work as the alternator(s) can self excite but on shutdown the engine cannot be restarted.. .. .Oh Lor' I wish this made it sound as simple as it is!. . . . <small>[ 13 March 2002, 21:18: Message edited by: P.Pilcher ]</small>